1. Field of the Invention
This invention relates to a tracking device provided with a linear motor, and, more particularly, it is concerned with a device for reciprocally moving a head radially along a disc-shaped record carrier to record and/or reproduce image information on the carrier.
2. Description of Prior Art
In an information and recording and playback device, particularly, a photo-disc device which records and reproduces information on a disc-shaped record carrier by use of optical means, an optical head, records and reproduces information signals to and from the record disc with high density. The optical head functions to form pits in the surface of the disc or to read out those pits already formed in the disc surface by irradiation of a light ray onto the disc while it is turning on a turn-table. With a view to enabling recording and reproducing of the information to be effected over the entire annular recorded region on the disc, the range of tracking of the optical head is required to extend over a long stroke corresponding to a radius of the disc. Further, in order to enable a rapid access to the recorded contents to be performed, the optical head is required to be moved at a high speed.
In addition to the above, in an ordinary photo-disc device, since a pit to be formed on the surface of the disc has a width of approximately 1 .mu.m, and a track has a pitch of approximately 2 .mu.m, it becomes a serious problem in the tracking of the optical head to avoid various mechanical errors such as deflections, vibrations, and so forth so as to achieve precise formation of the pits and accurate reading of the recorded information.
For desired tracking operations of the optical head, there has heretofore been used a system, in which the optical head is subjected to linear tracking operation in the radial direction of the disc by means of a driving device utilizing a linear motor.
FIG. 1 of the accompanying drawing illustrates a conventional device for the linear tracking of the optical head. A slider, on which the optical head is mounted, comprises a frame 1 constructed with four planar members 1A, 1B, 1C and 1D, a pair of rollers 2, 3 fixed on the inner surface of the frame 1, another pair of rollers 4, 5 resiliently fixed on the inner surface of the frame 1, and a winding 6 which is wound in a cylindrical form in the direction normal to the plane of the drawing. A yoke 7 passes through the inside of this cylindrical winding 6, while permanent magnets 8, 9 are disposed in mutual confrontation outside the winding 6 and in parallel with the yoke 7. Outside these permanent magnets, there are fixedly provided yokes 10, 11. These yokes 7, 10, 11 and the permanent magnets 8, 9 all have sufficient length in the direction normal to the drawing sheet, and constitute a linear motor together with the winding 6.
The yoke 10 has guide surfaces for the rollers 2, 3 formed in parallel with the surface of the disc (not shown in the drawing), while the yoke 11 has guide surfaces for the rollers 4, 5 with an inclination of 45 degrees with respect to the disc surface. Accordingly, the slider is sustained on the guide surfaces of the yoke 10 by means of the rollers 2, 3, and urged to the guide surfaces of the yoke 11 both upwardly and laterally by the rollers 4, 5.
In this conventional device, four other rollers are provided to roll on each of the guide surfaces, in addition to the above-mentioned four rollers 2 to 5, with the purpose of eliminating the pitching of the slider with respect to its tracking direction.
Since the conventional device is thus made up of a number of constituent parts such as the planar members to form the frame 1 for the slider, the rollers, and so on, the weight of the slider itself increases and its inertial mass becomes large, on account of which the high speed tracking of the optical head is adversely affected. Furthermore, since the slider has eight rollers urged toward the yokes from upper and lower directions, the combined frictional resistance between the rollers and yokes becomes naturally great. This frictional resistance increases when the rolling direction of each of the eight rollers does not accurately coincide, or when the direction of the inclined guide surfaces of the yoke 11 does not accurately coincide with the rolling direction of the rollers 2, 3 rolling on the yoke 10. The increase in the frictional resistance widens the stop position range of the slider to lower precision in positioning of the optical head.
Thus, in the conventional device, the slider is so constructed that it is urged to the guide surfaces of the guide rails from all directions by use of mechanical means to secure the linear tracking of the slider on and along the guide rails. On account of this, precision in the linear tracking of the slider depends strictly on the mechanical precision of each of the guide surfaces and the rollers.